Powered grips are used in materials testing to hold specimens. The specimen's tensile strength and failure properties can be tested by applying a tensile or compression force to the specimen that may be held by two spaced apart grips. The clamping force that may be generated by conventional powered grips used in materials testing is usually determined by the force generated by the source of power. Typically this force is proportional to the operating pressure in a pneumatic or hydraulic cylinder of the powered grip. To obtain different clamping forces, the pressure is adjustable typically down to 10-20% of the maximum system pressure. In addition, for a typical grip design, the pneumatic or hydraulic force generated may be multiplied by a fixed factor (assuming no mechanical loss) or ratio so that the required clamping force is achieved. This can be achieved via a wedge or lever system that offers a mechanical advantage.
The most common way of multiplying the pneumatic or hydraulic force is to use a wedge mechanism, a lever arm, a gear mechanism or sometimes by using a pressure intensifier.
In these known designs, the multiplication force is designed to be relatively constant. For example, in an established design of a grip, the pneumatic piston has a wedge to push cam rods resulting in a multiplication of the piston force. The cam rods may then push against a lever which multiplies the force further. Various other mechanisms exist but central to such designs is that the force multiplication is relatively constant, cannot be user adjustable and is simply for the purpose of creating a compact grip design with sufficient clamping force.
In grippers used not for materials testing, but for specimen handling, a similar design approach is in use. Here, designs often result in a more variable clamping force but such a characteristic is seen as a disadvantage where a larger stroke results in lower gripping force.
Accordingly it is an object of the present invention to provide an adjustable clamping force powered grip and/or a mechanism to allow adjustment of the clamping force of a powered grip that addresses the above mentioned problems and/or desiderata or that will at least provide the public with a useful choice.